1. Field of the Invention
The present invention relates to a product acceptance/rejection judgment method for an injection molding machine, and more particularly, to a product acceptance/rejection judgment method which makes it possible to accurately determine whether a product is acceptable or not at all times even when an injection molding machine is operated in various operating environments.
2. Description of the Related Art
It is known that a product sample taken out from a group of products that have been mass-produced by repeating a molding cycle is visually compared with a non-defective sample to determine whether it is acceptable or not. However, such visual inspection cannot ensure accurate judgment criteria, and also the non-defective sample changes on standing. For this reason, the product acceptance/rejection judgment based on visual inspection presents such problems as low reliability. Conventionally, therefore, it is judged whether or not the actual values of molding process parameters (cushion amount, injection time, metering time, metering completion point, injection pressure peak value, screw position where the injection speed control mode is switched to the injection pressure control mode, etc.) which are all related to the acceptance/rejection of a product are within tolerance limits during a molding cycle, and the product is determined to be defective if any of its molding process parameters is found to deviate from the tolerance limits.
However, when the injection molding machine is operated in different operating environments, there are some cases where defective units result even when molding cycles are implemented under molding conditions that should produce non-defective units, with all molding process parameters being within tolerance limits. This means that accurate product acceptance/rejection judgment may not be performed according to the aforementioned conventional method wherein the product acceptance/rejection judgment is made based on molding process parameters.
The major reason for this is considered that the conventional method does not adequately take into account operating environment changes involved in determining whether a product is acceptable or not. If the operating environments of an injection molding machine change, corresponding molding process parameter values change accordingly, therefore, the operating environment changes are indirectly considered in the conventional acceptance/rejection judgment. For instance, if a window of a factory which is usually closed is opened to let wind blow onto a die installed on an injection molding machine, causing the die to be cooled, then the injection time increases. And if the injection time exceeds its tolerance limit, then the product molded at that time is determined to be defective. However, it takes time for a molding process parameter value to deviate from its tolerance limit after a defective unit results from an operating environment change. During that time, therefore, accurate product acceptance/rejection judgment cannot be implemented, causing the occurrence of defective units.